Although the constancy of genetic information is central, genomes are surprisingly unstable. Transposable elements, i.e.discrete DNA segments that can move between many non-homologous positions, are widespread, having been found in virtually all organisms. Indeed, in some genomes, transposable element sequences are the predominantcomponent: strikingly, about 45% of the human genome is composed of transposable element sequences. The consequences of transposable element movement are many: insertion can result in alterations in host gene expression, in the covalent linkage of element-encoded determinates to host DNA, for example the joining of antibiotic resistance determinants to plasmids that can move between cells or the integration of retroviral DNA into host genomes. Also, gaps in the donor DNA followingelement excision are often not restored to their pre- transposon state. Moreover, DNA breaks can result in DNA damage signals. We propose to study at the molecular and biochemical level how transposition is controlled by its host and how the host responds to the movement of transposable elements, a potentially lethal event because it involves DNA breakage and joining. We will study the bacterial transposon Tn7 in E. coli and the insect transposon Hermes in S. cerevisiae and in Drosophila. We will probe how the assembly of particular nucleoprotein in complexes can control transposition though in vitro dissection of Tn7 transposition and will dissect the role of host functions in Tn7 transposition through the isolation and characterization of host mutants that alter transposition. We will also probe Hermes transposition in yeast and flies through the isolation of mutants that alter its movement. We will also examine how the transposase-mediated breaks can lead to chromosomal rearrangements, i.e. genetic instability.